1. Field of the Invention
The present invention relates to a developing apparatus adapted to develop an electrostatic latent image and provided in an image forming apparatus of electrophotographic or electrostatic recording type such as a copying machine, a printer and the like.
2. Related Background Art
In electrophotographic image forming apparatuses for forming an image on a recording medium by using an electrophotographic process, an electrostatic latent image on an image bearing member has been developed with toner.
Developing apparatuses for effecting development have seal members for preventing a developing agent (toner) from flowing out in a longitudinal direction at both ends of a developing sleeve for bearing the developing agent. In the past, the seal members for preventing the toner flow-out have widely been formed from an elastomer such as felt, foam rubber, or the like. An example is shown in FIGS. 10 and 11.
FIG. 10 is a side sectional view showing a main part of a conventional developing apparatus incorporated into a process cartridge in which an image bearing member and a developing apparatus for developing an electrostatic latent image on the image bearing member are integrally housed and which can be detachably mounted to an image forming apparatus, and FIG. 11 is a sectional view showing a main part of a developing agent bearing member (developing sleeve).
As shown in FIGS. 10 and 11, a developing sleeve 5 includes a magnet roller 6 therein and is rotatably supported within a developing container 18 via sleeve bearings 12. Accordingly, toner 3 supplied from the developing container 18 is adhered to a surface of the developing sleeve 5 by a magnetic force of the magnet roller 6. After a thickness of a toner layer is regulated to a constant value by a developing blade (developing agent amount regulating member) 7, as the developing sleeve (developing agent bearing member) 5 is rotated, the toner is adhered to an electrostatic latent image on a photosensitive drum (not shown) at a position where the developing sleeve is opposed to the latent image, thereby effecting development.
At both longitudinal ends of the developing sleeve 5 outside of a developing area, elastic seal members 8 are provided at a side opposite to an open side of the development sleeve 5 mounted within the developing container 18. By urging the elastic seal members 8 against an outer peripheral surface of the developing sleeve 5, the toner 3 is prevented from leaking outside.
However, in the developing apparatus having such a construction, since the seal members 8 are urged against the peripheral surface of the developing sleeve 5 along about a half thereof, a great load is applied to the rotating developing sleeve 5 during the development, and the elastic seal members 8 are deteriorated due to contact between the seal members and the developing sleeve 5, thereby worsening sealing ability.
Further, if the toner enters between the development sleeve 5 and the elastic seal member(s) 8, torque or torque fluctuation will be increased to cause uneven rotation, thereby effecting a bad-influence upon image formation.
To solve these problems, it is known to provide a technique in which magnetic seal members are arranged at both longitudinal ends of the developing sleeve 5 with a predetermined gap between the seal members and the developing sleeve to prevent the flow-out of the toner.
FIG. 12 is a side sectional view of a developing apparatus using magnetic seal members. In FIG. 12, the magnetic seal members 21 are formed from magnets (magnetic field generating means) and are arranged at both longitudinal ends of the developing sleeve 5 with predetermined gaps between the seal members and an outer peripheral surface of the developing sleeve 5. In this condition, the magnetic seal members and the developing sleeve 5 are attached to the developing container 18.
As shown in FIG. 13, for example, in each magnetic seal member 21, N poles and S poles are alternately arranged on an inner surface of the seal member, and a gap g between the outer peripheral surface of the developing sleeve 5 and a surface of the magnetic seal member 21 is closed by a magnet brush generated by chains of toner particles formed along lines 24 of magnetic force, thereby preventing the toner from flowing out in the longitudinal direction of the developing sleeve 5.
As another conventional example, as shown in FIGS. 14 and 15, it is known to provide a technique in which a magnetic brush is formed by using a magnetic seal member 21 having side surfaces magnetized to N poles and S poles, respectively, or a magnetic seal member 21 having front and rear surfaces magnetized to N poles and S poles, respectively.
By using such techniques, since the magnetic seal members 21 can be disposed not to contact with the developing sleeve 5, the rotation torque of the developing sleeve 5 is greatly reduced. Accordingly, a compact and a cheaper drive motor can be used, and, since fluctuation of rotation torque becomes smaller so that uneven rotations of the developing sleeve 5 and the photosensitive drum are unlikely to occur, reduction of image quality due to such uneven rotations can be prevented. Further, since the magnetic seal member 21 is not worn, the magnetic seal member can be used substantially permanently and can be recycled.
However, the magnetic seal member 21 is charged by voltage applied to the developing sleeve or frictional charges existing between the developing sleeve and the magnetic seal member, with the result that charges are accumulated on the magnetic seal member. When the magnetic seal member includes metal to reduce electric resistance as mentioned above, namely, when the magnetic seal member has a small capacitance, high potential may be generated on the surface of the magnetic seal member. As a result, a leak occurs between the developing sleeve and the magnetic seal member, or, when a member having great potential difference with respect to the magnetic seal member is disposed in the vicinity of the magnetic seal member, a leak occurs between the members. Due to noise generated by such leaks, erroneous operation of the apparatus may occur. Particularly, it was found that, when volume resistivity of the magnets of the magnetic seal member is 10.sup.5 .OMEGA.cm or less and a distance between the magnetic seal member and the adjacent member is 3 mm or less, the leak is apt to occur.